In accordance with the widespread use of portable electronic devices such as portable personal computers, handy video cameras and information terminals in recent years, non-aqueous electrolyte secondary batteries having a high voltage and a high energy density have been widely used as power sources. Furthermore, in view of environmental problems, battery automobiles and hybrid automobiles utilizing electrical power as a part of the power thereof have been put into practical use, and thus a non-aqueous electrolyte secondary battery having a high capacity is required.
Graphite, which is used in negative electrodes of non-aqueous electrolyte secondary batteries, had a problem that it generally has a higher capacity at a higher crystallinity, whereas a side reaction due to the reductive decomposition of an electrolyte which occurs on the surface of the graphite increases as the crystallinity becomes higher. When this side reaction increases, a negative electrode capacity decreases due to the peeling of crystal layers of graphite particles, or a decomposed product due to the side reaction accumulates on the surface of the negative electrode and the internal resistance of the surface of the negative electrode increases, thereby the battery performance decreases. Therefore, in order to prevent the peeling of the crystal layers of graphite particles, countermeasures such as (i) “a method using graphite particles having a surface coated with a noncrystalline carbon material as a negative electrode active material”, (ii) “a method using optical anisotropic small spherical bodies (mesocarbon microbeads: MCMB) that are obtained upon heating of pitches at around 400° C. as a negative electrode active material”, (iii) “a method including kneading, calcining and pulverizing a binder that can be graphitized with microcrystalline graphite, and using the product as a negative electrode active material”, and (iv) “a method using an electrolyte additive that protects a negative electrode in an electrolyte” have been made.
As the above-mentioned (i), for example, Patent Literature 1 enables use of graphite having a high crystallinity as a negative electrode active material by covering the surface with amorphous carbon by a chemical vapor deposition process.
As the above-mentioned (ii), for example, Patent Literature 2 discloses a method using “a graphite powder, to which lithium can be intercalated, which is a spherical substance, and also an optically anisotropic granular substance having a lamella structure formed of a single phase, and this granular substance is formed by graphitizing mesophase small spherical bodies generated in a process of a heat treatment of pitch at a low temperature, wherein the graphite powder has a spacing of a 002 face (d002) by a wide angle X-ray diffraction method of from 3.36 to 3.40 Angstrom and a specific surface area by a BET method of 0.7 to 5.0 m2/g” as a negative electrode active material.
As the above-mentioned (iii), for example, Patent Literature 3 discloses a method using “a powdery carbon material, which is a mixture of generally-spherical graphite particles (A) each having a minute projection on the surface, and obtained by impregnating and coating a base material formed by shaping clove-like graphite into a spherical shape with a mixture of pitch and carbon black and baking it at 900 to 1,500° C., and carbonaceous particles (B) obtained by baking a mixture of pitch and carbon black at 900 to 1,500° C., and thereafter crushing and sizing it, and has composite peaks of a G band having peaks in the vicinity of 1,600 cm−1 and in the vicinity of 1,580 cm−1, and at least one peak in the vicinity of 1,380 cm−1 of a D band in a Raman spectrum spectral analysis using argon laser Raman light having a wavelength of 514.5 nm, and having a poly-phase structure having a spacing d002 of a crystal plane obtained by X-ray wide-angle diffraction of 0.335 to 0.337 nm” as a negative electrode active material.
The above-mentioned (i) has a problem that the surfaces of the graphite particles are not exposed, and thus the cost increases in the step for coating the surfaces with graphite having a low crystallinity; the above-mentioned (ii) has a problem that the cost increases in the step for removing the MCMB from the pitch; and the above-mentioned (iii) has problems from the viewpoints of cost and processes since a treatment is conducted by using pitch. In the above-mentioned (ii) and (iii), it is essential to conduct a pretreatment, and a process to make the orientation planes of the graphite particles used as a negative electrode active material non-parallel is conducted.
Namely, in the case when graphite particles having a high crystallinity are used as a negative electrode active material, a countermeasure to make the orientation planes of the graphite particles non-parallel, or to coat the surfaces of the particles with a carbon material having a low crystallinity or the like, namely, to treat the crystal faces of the particles to prevent the faces from being exposed, has been made.
Furthermore, as the electrolyte additive that protects a negative electrode in the above-mentioned (iv), for example, 1,3-propanesultone, vinylethylene carbonate, 1,3-propanesultone, butanesultone, vinylene carbonate, vinylethylene carbonate and the like are known, and among these, vinylene carbonate is widely used since it is highly effective. For example, Patent Literature 4 discloses “a lithium secondary battery formed of a positive electrode, a negative electrode containing a carbon material as a negative electrode material, and an electrolyte in which an electrolyte is dissolved in a non-aqueous solvent, wherein the above-mentioned non-aqueous solvent contains propylene carbonate, a chain carbonate and vinylene carbonate”.
It is considered that the above-mentioned electrolyte additive forms a stable coating that is called as an SEI (Solid Electrolyte Interface) on the surface of the negative electrode, and this coating covers the surface of the negative electrode and suppresses the reductive decomposition of the electrolyte to thereby protect the negative electrode.
However, a sufficient effect could not be obtained when the electrolyte additive was used in a non-aqueous electrolyte secondary battery having a negative electrode using graphite particles in which the graphite particles are bonded with each other so as to be parallel to the orientation plane of each other, i.e., the orientation planes of the particles have not been subjected to a non-parallelization treatment in a negative electrode active material, and thus the non-aqueous electrolyte secondary battery could not be stably used for a long period. In the case when the electrolyte additive is excessively added so as to compensate this disadvantage, a problem that the thickness of a coating as formed increases and the resistance elevation rate increases to conversely cause decrease of the battery performance is caused, and thus addition of these electrolyte additives to an electrolyte was insufficient to improve the balance of both of the output and capacity of the battery.
Therefore, in a non-aqueous electrolyte secondary battery having a negative electrode containing graphite particles as a negative electrode active material, a positive electrode containing a lithium-containing oxide of a transition metal or a lithium-containing phosphate of a transition metal as a positive electrode active material, and a non-aqueous electrolyte in which a lithium salt is dissolved in an organic solvent, wherein the graphite particles have an exposed crystal face and are bonded with each other so as to be parallel to the orientation plane of each other, both of the output and capacity of the battery were not at sufficient levels, and thus a non-aqueous electrolyte secondary battery that improves the balance thereof has been desired.
Patent Literature 5 and Patent Literature 6 disclose “an electrochemical cell, which contains a negative electrode that intercalates with an alkali metal, a positive electrode containing an electrode active material that intercalates with the alkali metal, a non-aqueous electrolyte that activates the negative electrode and positive electrode, and a phosphate additive that is added to the electrolyte, wherein the phosphate additive is represented by the general formula: (R′O)P(═O) (OR2) (OR3), wherein R1, R2 and R3 are the same or different, and at least one, but not all three, of the R groups is a hydrogen atom, or at least one of the R groups has at least 3 carbon atoms and contains an sp or sp2 hybridized carbon atom bonded to an sp3 hybridized carbon atom bonded to the oxygen atom bonded to the phosphorous atom, and that the negative electrode may contain a carbon material (for example, coke, carbon black, graphite, acetylene black, carbon fibers, glassy carbon and the like) as a negative electrode active material”.
However, Patent Literature 5 and Patent Literature 6 do not disclose at all the effect and preferable conditions for use in the case when the above-mentioned phosphate additive is used in a negative electrode containing graphite particles having a high crystallinity, each having an exposed crystal face and are bonded with each other so as to be parallel to the orientation plane of each other as a negative electrode active material.